An experimental murine model of malignant lymphomas, the chronic GVHR, will be studied immunologically and virologically. The mechanism of production of F1 host tumors after inoculation of immunocompetent parental grafts will be analyzed. The participation of the host in the immunopathologic lesions of the chronic GVHR will be investigated. The role of idiotypes in the progressive lymphoproliferative disorder of the chronic GVHR will be stressed. An alternative hypothesis, that the chronic GVHR is an autoimmune disease triggered by parental T cells, will also be tested. The lymphomas will be classified by cell markers and antigen-binding properties. The role of C-type RNA viruses will be examined and clarified, with particular emphasis on: (a) susceptibility of antigen-stimulated lymphocytes to ecotropic viruses and (b) potential oncogenicity of xenotropic viruses in the host of origin.